The present invention relates to a lockup control system for setting a torque converter installed between an internal combustion engine and an automatic transmission into a lockup condition and, more particularly, to a lockup control for suppressing vibrations during a coast lockup which is caused by releasing an acceleration pedal in a lockup range.
A torque converter installed in a transfer system of an automatic transmission is generally arranged to transmit a power from an input element to an output element through fluid. Therefore, the torque converter has a torque fluctuation absorbing function and a torque amplifying function, but degrades a transmission efficiency. In order to improve such transmission efficiency, a torque converter with a lockup clutch have been widely used since it is arranged to establish a direct connection between an engine and the automatic transmission when the vehicle is put in a lockup range where the torque fluctuation absorbing function and the torque amplifying function are not required.
If the lockup torque converter with the lockup clutch is put in a coast lockup range and only if an engine rotation speed or a vehicle speed is low, the lockup torque converter tends to generate undesired vibrations. More specifically, during the lockup proceeding process the engagement capacity of the lockup clutch tends to be excessively large due to the decrease of the engine power caused by releasing the acceleration pedal. This causes a radical engagement of the lockup clutch, thereby generating torque fluctuations and shocks. Further, in a lockup established condition, the decrease of the engine output due to the release of the acceleration pedal directly generates the shocks and vibrations due to torque fluctuation since the torque converter put in the lockup condition is not able to absorb such torque fluctuation.
A Japanese Patent Provisional Publication No. S61-99763 has proposed a system for improving the above-mentioned shocks and vibrations during the coast lockup.